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From: Andrew Thompson <thompsa@FreeBSD.org>
To: Tom Judge <tom@tomjudge.com>
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User-Agent: Mutt/1.5.16 (2007-06-09)
Cc: freebsd-net@freebsd.org, "Bruce M. Simpson" <bms@FreeBSD.org>
Subject: Re: Programming interface MAC filter without enabling PROMISC on
	an interface from user space.
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On Tue, Jan 15, 2008 at 09:59:22AM +0000, Tom Judge wrote:
> Bruce M. Simpson wrote:
>> Tom Judge wrote:
> <SNIP>
>>> Personally I can't see why this approach would be a problem,  but I am 
>>> not a expert.  The address is defined in IEEE Std 802.1D-2004 as to not 
>>> be forwarded by bridges (which I interpret as it being link local in a 
>>> sense as switches/bridges are not allowed to forward the frame), so I 
>>> can't see it being a problem registered on multiple interfaces.
>> SIOCADDMULTI memberships are specific to the interface you request them 
>> on. I can't speak for the bridging code -- I don't think it does any 
>> special handling of multicast frames, however I'm not sure if it's smart 
>> enough not to forward this group. Like IN_LOCALGROUP() it might need its 
>> own 'don't forward this' clause.
>
>
> Just for the record it seems that if_bridge replaces the destination MAC of 
> a Ethernet multicast packet with its own MAC therefore making sure that the 
> packets are not forwarded.  Andrew can you confirm this assumption?  (Based 
> on sys/net/if_bridge.c lines 2011-2018 on RELENG_6_2)

No, the only multicast address that the bridge does not forward is the
STP one (01:80:c2:00:00:00). It will pass LLDP frames.


Andrew